Method and apparatus for measuring particle size

ABSTRACT

The disclosure relates to an apparatus for measuring the size of gas entrained particles by detecting the rate of incandescent burning of the particles. Entrained particles are passed at a controlled velocity through a heated zone where they are gradually consumed. The radiant energy emitted by the burning particles is detected at varying locations along the path of flow through the heated zone. An indication of particle size distribution is obtained by deriving the third derivative of detected radiant energy intensity with respect to distance of flow through the heated zone.

BACKGROUND OF THE DISCLOSURE

The present invention relates to method and apparatus for measuring thesize of gas-suspended particles. Preferred embodiments of the inventionrelate to the measurement of the size distribution of carbonaceousparticles such as soot or combustion products found in exhaust gases.

Measurement of gas-suspended particles is a subject of growing interestin the field of emission control. A convenient, accurate and inexpensivemethod for measuring emission particle size distribution is needed inthe study of the effects and control of particulate emissions.

In the past, a number of different methods have been taught for themeasurement of the size of gas-suspended particles. It has been proposedthat particle size be measured by diffusion of light. Such a method isdiscussed in U.S. Pat. No. 3,825,345 to Lorenz, which notesdisadvantages in this technique due to the complex functionalrelationship between particle size and the signal obtained, and thelimitations of the technique due to an unfavorable signal to noise ratiofor small particles. Somewhat similar techniques involving the detectionof the effect of particles on light directed at them, rather thandetecting light emitted by the particles, are taught in U.S. Pat. No.3,680,961 to Rudd and U.S. Pat. No. 3,851,169 to Faxvog.

A second technique involves measurement of the thermal emission ofparticles by exposing the particles to a high temperature hydrogenflare. This method is also discussed in U.S. Pat. No. 3,825,345 toLorenz where it is noted that, using this second technique, particlesare singly and consecutively taken to a minute hydrogen flare in whichthey are evaporated. The particles are said to emit a luminescent flashof an intensity proportional to their mass. Lorenz indicates that thisprocedure is unsuitable for particle size analysis of atmosphericaerosols and often lacks sensitivity.

Lorenz proposes the use of an atomic absorption spectometer having anevaporating zone small enough to be completely filled by a singleevaporated particle. However, in addition to the expense associated withthe provision of an atomic absorption spectometer, the method isconstrained to detecting particles, one at a time rather than detectingpopulations of gas suspended particles simultaneously.

U.S. Pat. No. 3,790,282 to Fielding teaches a method of determining theconcentration of pollutants in air by exciting atoms of the pollutant byburning the pollutant with a fuel in a pressure chamber and measuringthe intensity of the "characteristic light" emitted by the excitedatoms. The Fielding patent indicates that from this intensity, theconcentration of the pollutant can easily be calculated. The Fieldingpatent does not teach the measurement of size distribution of thepollutant particles.

It has been proposed to measure gas-suspended particle size distributionby the method of cascading inertial impactors. In such method a seriesof plates are positioned to intercept a gas stream carrying particles.Each plate is positioned behind an opening of a given size. The openingsthrough successive plates are progressively smaller so that particlesinertially adhering to each plate after impact are of different sizeranges. After exposure of the plates to a gas-suspended particle streamfor a given time, the plates are weighed. The measured weights areplotted against the plate opening size to indicate size distribution ofthe particles. The method has the disadvantages that it requires longmeasurement times and does not yield a continuous distribution.

Other art, of more general interest, is found in the following patents:U.S. Pat. No. 3,088,808 to Mandell, Jr.; U.S. Pat. No. 3,518,001 toHell; U.S. Pat. No. 3,700,330 to Davis; U.S. Pat. No. 3,740,149 toWhetten; U.S. Pat. No. 3,860,345 to Raillere et al; U.S. Pat. No.4,021,117 to Gohde et al; and U.S. Pat. No. 4,279,512 to Tunstall.

Accordingly, it is a primary object of the present invention to providea novel, convenient, accurate and inexpensive method and apparatus formeasuring gas-suspended particle size distribution.

It is another object of the present invention to provide a novel methodand apparatus for measurement of the continuous size distribution ofcarbonaceous particles suspended in a gas such as an exhaust stream.

It is another object of the present invention to provide a novel methodand apparatus to rapidly measure the size distribution of simultaneouslydetected populations of particles suspended in a gas.

These and other objects and features of the invention will becomeapparent from the claims, and from the following description when readin conjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention relates to a method and apparatus for measuringthe size distribution of particles entrained in a gas. Moreparticularly, the invention relates to a method and apparatus formeasuring the continuous size distribution of gas-suspended carbonaceousparticles through induced dynamic burnout incandescence. The measurementis made by first passing the particles and entraining gas through aheated zone to induce incandescent burning of the particles whichresults in the particles entrained in the gas being gradually consumedas the particles move through the heated zone. The radiant energyemitted by the burning, entrained particles is detected at varyinglocations along the path of flow of the gas through the heated zone. Anindication of the size distribution of the particles entrained in thegas is produced responsive to the rate at which the detected radiantenergy varies with the detection location along the path of flow.

In a preferred embodiment of the present invention an apparatus isemployed which includes a burning chamber. A conduit is provided forintroducing gas-suspended carbonaceous particles through an inlet intothe burning chamber, whereupon the gas passes through the burningchamber at a controlled velocity. The burning chamber is heatedapproximately uniformly along the path of flow of the gas to induceincandescent burning of the particles as the gas passes through theburning chamber. A detector is provided for detecting the radiant energyintensity of the heated particles in a detection zone within the burningchamber. The detector provides a first signal responsive to the detectedradiant energy as the distance between the detection zone and the inletis varied. In a preferred embodiment the distance variation is effectedby a screw drive which moves the burning chamber housing relative to aconduit inlet. A sensor is provided which produces a second distancesignal functionally related to the distance between the detection zoneand the inlet. A circuit responsive to the first and second signalsproduces an output signal functionally related to the third derivativeof detected radiant energy intensity with respect to the distancebetween the detection zone and the inlet as an indication of the sizedistribution of the particles.

The gas-suspended particles may be introduced into the apparatus bysampling gas-suspended carbonaceous particles from the stream such as astream of combustion product. A conduit is provided for maintaining thesampled, gas-suspended particles at a temperature below the burningtemperature of the carbonaceous particles prior to introduction into theburning chamber through the inlet. Oxygen may be added to the sampled,gas-suspended particles prior to introduction into the burning chamber.

In a preferred embodiment, the photodetector may include a cooled mirrorextending around the periphery of the detection zone; a photomultipliertube optically exposed to the mirror and detection zone; and baffles foroptically shielding the photomultiplier tube from exposure to surfacesof the burning chamber.

In a preferred embodiment of the present invention, the indication ofthe size distribution of the particles is produced by a circuitincluding differentiating circuits and dividing circuits which processthe photodetector signal and the distance signal in order to produce anoutput signal functionally related to the third derivative of thedetected light intensity with respect to the distance between thedetection zone and the inlet. An X-Y recorder may be provided in whichthe distance signal controls deflection along one axis and the outputsignal from the processing circuit controls deflection along the otheraxis, whereby a graphical recording is made as an indication of the sizedistribution of the particles. Further aspects and details of thepresent invention will be apparent from the detailed description givenbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram in partial cross-section of an apparatusfor measurement of the continuous size distribution of gas-suspendedparticles, according to a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional pictorial view of a gas handling anddetector assembly employed in an embodiment of the present invention.

FIG. 3 is a cross-sectional view of the gas handling arrangement of FIG.2 taken in the direction of arrows 3--3.

FIG. 4 is a schematic diagram of a processing circuit and recorderarrangement employed in an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention employs the technique of measuring light emittedby burning particles, in such a way that size distribution of theparticles can be determined therefrom. The burning particles decrease insize as they burn with a consequent reduction in light output. It isthis reduction of intensity with time which provides the functionalbasis for producing indications of particle size distribution.

The following mathematical derivation will help to clarify the operationof the present invention.

For purposes of this derivation the following assumptions are made:

1. The gas-suspended particles whose size distribution are to bemeasured are spherical.

2. The rate at which a particle's volume decreases by burning isproportional to its surface area.

3. Particle burning does not significantly affect its temperature orradiant emission.

The first assumption is good for fresh combustion exhaust. The secondassumption should be good for particles which are large in proportion tothe boundary layer of exchanging gases around them. Finally, the thirdassumption can be made good by carefully controlling the conditions ofthe measurement.

The derivation is developed by establishing the radiant intensity as afunction of time of a particle which is decreasing in size by burning.This functionality is used to establish the relationship describing theradiant intensity as a function of time from a given distribution ofdifferent sized particles. As will be discussed, this relationship maybe used to extract particle size distribution information from lightintensity measurements.

The time rate of change of light from a burning particle is proportionalto the time rate of change of its surface area: ##EQU1## where l islight intensity, α is the proportionality constant, and r is the radiusof the spherical particle.

If a spherical particle's volume decreases at a rate proportional to itssurface area, (a reasonable approximation for burning) then: ##EQU2##where V is the particle volume and β is the proportionality constant.But from the volume formula for a sphere: ##EQU3## Integrating andimposing r=r_(o) at t=o yields

    r=r.sub.o -βt                                         (5)

Substituting Equations (4) and (5) into Equation (1) yields:

    dl=-8αβπ(r.sub.o -βt)dt                 (6)

Integrating this expression and scaling α such that l=4απr_(o) ² at t=oyields: ##EQU4## If P(r_(o)) is the probability function describing thedistribution of particle radii, then the cumulative light intensity I(t)at time t from the particles remaining from the original distributionis: ##EQU5## From the second time derivative of Equation (8): ##EQU6##P(r_(o)) can be found by differentiating Equation (9) with respect tor_(o) ; and by recognizing that r_(o) =βt at the moment a particle ofradius r_(o) vanishes [from Equation (4)], it follows that: ##EQU7##This is the relationship which equates the desired particle sizedistribution to the measurable radiant intensity as a function of time.

The following describes an apparatus which serves to measure theforegoing intensity distribution.

By passing gas-suspended particles to be analyzed through a tube at aknown or controlled velocity v, the intensity of particle radiation as afunction of the distance, x, from the entry point is equivalent to theintensity function of Equation (10): ##EQU8## Thus, the probabilityfunction describing the distribution of particle radii can be determinedby measuring the intensity I(x) and obtaining its third derivative.

An apparatus suitable for measurement of I(x), according to a preferredembodiment of the present invention, is shown schematically in partialcross-section in FIG. 1. The apparatus includes a gas handling anddetector assembly 10 and processing circuits and recorder 12 and 14,respectively.

In a preferred embodiment of the present invention a sample ofgas-suspended particles may be withdrawn from a gas stream 16 such as anexhaust stream carrying combustion products or soot. The sampledfraction is drawn into conduit 18 where it is combined with oxygenintroduced through oxygen inlet 20. The oxygen flow rate may be selectedto optimize the burning rate of all particles. Advantageously, theconduit 18 is made of a good heat conducting material and is cooled by awater cooling coil 22 to maintain the sampled gas fraction at arelatively low temperature at which the gas-suspended particles do notburn. At an end of the conduit 18, one or more openings are providedwhich serve as an inlet 24 to a burning chamber 26. Advantageously, flowthrough inlet 24 is non-turbulent to facilitate a uniform treatment ofall particles entering the chamber. The construction and arrangement ofthe burning chamber, inlet port and conduit are discussed in greaterdetail below in connection with FIG. 2.

The burning chamber 26 is provided with heating coils 28 which areselected and configured to raise the temperature in the burning chambersufficiently to cause incandescent burning of the gas-suspendedparticles. Gas is exhausted from the burning chamber by a blower 29. Theburning chamber temperature is selected so that the heat generated inburning the particles in the burning chamber does not provide unwantedvariations in the particle temperature.

A radiant energy detector assembly 30 is provided for detecting theintensity of radiant energy (typically visible light and/or infraredenergy) from heated particles in a detection zone within the burningchamber and for providing a first signal responsive thereto. This firstsignal is applied to a first terminal A of the processing circuits 12.

As indicated by the double headed arrow 32 of FIG. 1, the inlet 24 ofthe conduit 18 is movable within the burning chamber 26, while theradiant energy detector 30 is stationary with respect to the burningchamber. The relative motion indicated by the double headed arrow 32 maybe imparted by a screw drive mechanism indicated generally by thenumeral 34. As the screw of the screw drive is rotated, an upper portionof the conduit 18 slides into or out of the burning chamber 26.

Relative movement of the inlet 24 with respect to the burning chamber issensed by mechanical or electrical means indicated by the dotted line36. This sensed movement is converted to an electrical signal employedby the processing circuits 12, as will be discussed in greater detail inconnection with FIG. 3 below.

The processing circuits 12 produce an output signal, indicated by arrow38, which is functionally related to the third derivative of detectedlight intensity with respect to the distance between the inlet 24 andthe region of the burning chamber to which the photodetector assembly isdirected. This output signal is an indication of the size distributionof the particles and may be applied to the chart recorder 14 to providea graphical indication of the size distribution of the particles.

FIG. 2 is a cross-sectional pictorial view of gas handling and detectorassembly in which structure similar to those of FIG. 1 are indicated bylike numerals. The gas handling and detector assembly, as depicted inFIG. 2, includes a portion of the conduit 18 for introducinggas-suspended particles into the burning chamber 26. The detectorassembly, denoted generally by the numeral 30, is attached to theburning chamber 26.

As shown in the FIG. 2, the conduit 18 is formed with plural passages 39through which the gas-suspended particles pass. Advantageously, the flowof the particles into the burning chamber is laminar as discussed above.Within the conduit, the gas-suspended particles are maintained at atemperature T₁, selected so that the particles of interest do not burnprior to entry into the burning chamber. The passages terminate ininlets 24 for the burning chamber. The inlets constitute widenedportions of the passages in the conduit 18 which accommodate theexpansion of gas due to the heat provided in the burning chamber.

The conduit 18 is slideably mounted within the burning chamber by asliding seal 40 to permit relative movement of the conduit with respectto the burning chamber and limit leakage of the gas-suspended particles.

The inner wall of the burning chamber is placed in thermal contact witha suitable heating means 28, for example an electrical resistanceheating element such as a flexible, elongated electrical resistance wireencapsulated in a coaxial sheath. As indicated in FIG. 2 the heatingelement heated zone 42 in the burning chamber to temperature T₂.Temperature T₂ is a temperature sufficient to cause incandescent burningof the gas-suspended particles and is, advantageously, selected to behigh enough so that the burning of the gas-suspended particles does notsignificantly increase the temperature within the burning zone 42.

The detector assembly 30 may include a cooled mirror 44 attached toexterior housing 26, which is shielded by heat shield 27 from thetemperature and burning chamber 26 by concentric metal foil shieldsextending around the periphery of burning chamber 26. This arrangementis also shown in FIG. 3, which is a cross-sectional view of theapparatus of FIG. 2, taken in the direction of arrows 3--3 and in whichlike structures are identified with like numerals. The detector assembly30 also includes photomultiplier tube 46.

In front of photomultiplier tube 46 lies a light filter 48 and a baffledtube 50 for collimating radiant energy from the heated zone 42. Radiantenergy from the heated zone 42 passes to the cooled mirror 44 via acircumferential aperature 52 in the heated chamber. The opticalparameters of the detector define a detection zone 54 within the heatedzone 42. Radiant energy from the incandescent burning of particleswithin the detected zone 54 radiates to the circumferential mirror 44and tube 50 through the aperature 52. The light is collimated andfiltered by filter 48 for selecting radiation yielding an optimum signalto noise ratio. The light finally impinges on the photocathode 56 of thephotomultiplier tube. The arrangement of the mirror 44, aperatures 52and baffle tube 50 is configured to optically shield the photocathode ofthe photomultiplier tube from exposure to surfaces of the heated chamberto exclude emissions from such surfaces and limit the radiant energydetected to the radiant energy emitted by the burning particles withinthe detection zone 54.

Radiant energy impinging on the photocathode 56 is amplified through acascading action of electrons along the consecutive dynodes of thephotomultiplier tube. An output signal related in value to the intensityof the radiant energy in the detected zone 54 appears at the outputterminal a of the photomultiplier tube.

Referring now to FIG. 4, an embodiment of the processing circuits 12 andrecorder 14 is described in detail. For the sake of clarity, the presentinvention is described in terms of an analog electrical system ofamplifiers and divider circuits. It will be obvious that in actualpractice microprocessors or other digital computer means may be used toperform each of the arithmetic functions required to determine theresults of the size distribution measurements.

The processing circuit 12 shown in FIG. 4 may receive an outputelectrical signal at terminal A from a photomultiplier tube such as thatdescribed in connection with FIG. 2. The processing circuit 12 may alsoreceive an electrical or mechanical signal indicative of the distance xbetween the inlet 24 and detection zone 54. See FIG. 2. The particularcircuit shown in FIG. 4 receives a mechanical signal functionallyrelated to the relative positions of the inlet 24 and the detection zone54 of FIG. 2. The mechanical signal, indicated schematically by dashedline 36 in FIG. 4, corresponds to a mechanical coupling of therelatively moving parts of the gas handling and photodetector assemblyto a variable resistor 60. A supply voltage 62 is applied across thefixed terminals of the variable resistor 60 and a signal obtained atnode 64 is proportional to the distance x between the inlet 24 and thedetection zone 54.

The output signal of the photomultiplier tube, applied at terminal A ofFIG. 4, is electrically amplified by a preamplifier 66. This signal isthen differentiated by first differentiation circuit 68. An outputsignal of the first differentiation circuit 68 is applied to an inputterminal of a first divider circuit 70. In a similar manner, the seconddifferentiating circuit 72 differentiates the electrical signaloccurring at node 64. An output signal of the second differentiatingcircuit 72 is applied as a divisor input signal to the first dividercircuit 70. An output signal of the first divider circuit 70 is appliedto a third differentiating circuit 74. An output signal of the thirddifferentiating circuit 74 is applied to a second divider circuit 76.Likewise, the output signal of the second differentiating circuit 72 isapplied as the divisor to the second dividing circuit 76. An outputsignal of the second divider circuit 76 is applied to a fourthdifferentiating circuit 78. The output signal of the fourthdifferentiating circuit 78 is applied to a third divider circuit 80. Theoutput signal of the second differentiating circuit 72 is likewiseapplied as the divisor to the dividing circuit 80. Finally, an outputsignal of the third divider circuit 80 is applied to the chart recorderindicated generally by the numeral 14.

In a preferred embodiment, a signal from node 64 may be applied tocontrol deflection along the x axis of the chart recorder 14, and theoutput signal of the divider circuit 80 may be applied to control thedeflection along the y axis of the chart recorder 14. Application of theoutput signals from the processing circuit 12 in the manner shown inFIG. 3 may be used to provide a graphical representation of the sizedistribution of particles entrained in the gas introduced into theburning chamber. The deflection of the recorder along the y axis isproportional to the number of particles of a particular size and thedeflection along the x axis is indicative of the radius of the particlesmeasured.

Although the invention has been described with preferred embodiments, itis to be understood that variations and modifications may be resorted toas will be apparent to those skilled in the art. Such variations andmodifications are to be considered within the purview and the scope ofthe claims appended hereto.

What is claimed is:
 1. A method for measuring the size distribution of carbonaceous particles entrained in a gas comprising the steps of:passing the entraining gas through a heated zone to induce incandescent burning of the particles, whereby the particles entrained in the gas are gradually consumed as the particles move through the heated zone; detecting a portion of the electromagnetic energy emitted by the burning, entrained particles at varying locations along the path of flow of the gas through the heated zone; and producing an indication of the size distribution of the particles entrained in the gas responsive to the rate at which the detected electromagnetic energy varies with detection location along the path of flow.
 2. A method for measuring the size of incandescently burnable particles suspended in a gas comprising the steps of:heating the particles to a temperature sufficient to cause incandescent burning of the particles; detecting the intensity of radiant energy emitted by heated burning particles; and providing an output signal, responsive to the detected intensity of radiant energy emitted by the burning particles, related in value to the third derivative of the intensity of emitted radiant energy with respect to time, said output signal providing an indication of the size of the detected burning particles. 